In the conditioning and controlling of the temperature of molten glass flowing through a forehearth, it is typical that such forehearths have a series of gas burners arranged above the level of the glass at either side. The burners have their flames directed just above the surface of the glass, the effect of which is to control the amount of heat loss from the glass to the forehearth environment and atmosphere.
It is important that the glass temperature and viscosity at the time that it issues from the forehearth be closely controlled. The feeding of molten glass from the forehearth is in a stream that is divided into gobs or charges of glass in glass container manufacturing. In the melting and feeding of colored glass, it is much more difficult to control the temperature because the flame burning over the surface of the glass does not penetrate by radiation and conduction as readily as would be the case in the feeding of clear or flint glass.
The glass entering the forehearth comes from a refiner where its exit temperature is generally controlled so that the temperature of the glass adjacent the spout of the forehearth may be predicted. On standard gas heated forehearths the glass along the side of the channel runs colder than the glass in the center of the channel. With colored glasses, this normally would lead to a side temperature being 40.degree. to 60.degree. F. cooler than the center line temperature. In some instances, these side-to-center differences in temperature have been as high as 120.degree. F. This temperature imbalance has a marked effect on the glass distribution in the containers made from the glass. As production speeds are increased and the bottle weights are decreased, the amount of the temperature gradient that can be tolerated becomes crucial.
Typically, thermocouples or other temperature measuring devices immersed in the glass have been used at the refiner exit or forehearth entrance. Furthermore, thermocouples have been positioned at various points along the length of the forehearth. The portion of the forehearth which is immediately next to the spout or feeder is termed the "conditioning section" of the forehearth and it is to this section of the forehearth which the present invention is specifically addressed.
Electrical heating of glass forehearths, of course, is not a new consideration and, as a recent example, reference may be had to U.S. Pat. No. 4,227,909 dated Oct. 14, 1980 and issued in the name of Hornyak, Jr., et al. This particular patent discloses an arrangement for providing joule effect heating across the channel of the forehearth by positioning of the electrodes of opposite polarities on opposite sides of the forehearth. In addition, the patent teaches an arrangement for assuring that all of the electrodes in the forehearth are kept at the same electrical potential relative to a counter-electrode immersed in the glass melter.
Another patent recently issued, U.S. Pat. No. 4,247,733 to Sevenson dated Jan. 27, 1981, discloses an electrically heated glass forehearth in which the electrodes appear to extend across the full width of the forehearth, with adjacent electrodes being of different polarity to provide a joule effect heating current therebetween. These electrodes are such as to divide the forehearth into a plurality of individual zones under separate control.